The invention relates to an apparatus for reducing overload and dampening collision energy acting on lifting ropes of a crane, the apparatus comprising movable means for receiving a load, and resistance means against the predetermined resistance of which the movable means are able to move in a dampening situation, arranged in connection with the lifting ropes of the crane.
Problems caused by overload and collision energy occur, for instance, in connection with container cranes, a typical problematic situation arises when a lifting element is suddenly and unexpectedly caught to a ship to be unloaded during a lifting movement. Another serious situation arises when the lifting element is run at full speed to collide with the lower part of a lifting carriage. This is possible if the control limit of the lifting movement fails to operate.
Damage prevention in connection with cranes is represented by a prior art arrangement wherein a lifting rope of the crane is supplied through a sheave in such a way that a hydraulic cylinder can be placed as a support in the angle formed by the ropes. The pressure required by the rope power is achieved with a machine unit that operates all the time when the crane is employed. On collision, the kinetic energy is converted to heat by means of throttle valves, when a cylinder piston pushes oil ahead. Oil pressure is determined according to the adjustment of the throttle valve. The hydraulic machine unit resets the cylinder to its original operating position. The solutions disclosed in U.S. Pat. No. 5 597 080 and EP 0 358 343 A1 can be given as examples of the prior art technology. Drawbacks of these solutions based on hydraulics are a high purchase price and a constant need for maintenance. Oil leaks may also occur.
Other known applications are mostly buffers operating on hydraulic fluid. Their use is restricted by the fact that buffers cannot be loaded with a continuous rated load, and only reduce the load exceeding the rated load. Buffers of this kind have no separate machine unit. On the other hand, buffers made of polyurethane or a similar material, are very large at high energy levels, and thus in most cases, they do not fit in the structure of the application.
Mechanical solutions are also known, such as for instance, an implementation presented in German Patent Publication DE 44 22 927 A1, in which overload is converted to kinetic and/or electric energy while releasing the lifting ropes of a crane. This technique is also complicated and expensive.